In an automobile radar device using millimeter wave signals or in a high frequency wireless communication device or wireless terminal device using radio wave signals in a frequency band of 300 MHz or higher, a box thereof is so structured as to contain a single multi-function semiconductor element or integrated circuit (IC), a package comprising a plurality of ICs, or a high frequency circuit element comprising a plurality of mutually connected ICs including circuits for filtering and other functions for the purpose of realizing reduction in size and cost and incorporating multiple circuit functions.
As an example of a communication device having such a structure as mentioned above, there is an automobile radar transmitter-receiver reported in the Proceedings of the 1997 Institute IEICE Conference C-2-121 “60 GHz Band Millimeter Radar Unit”. This device is structured to contain a millimeter wave (60 GHz band) transmitter-receiver circuit in a box thereof having plane internal surfaces. As another example, there is an RF subsystem found at the lower part of the front page photograph of the Catalog “RF, Microwave and Millimeter Wave, Single and Multi-Function Components and Subassemblies” of M/A-COM Co. U.S.A., issued in 1996. This RF subsystem is so designed that a plurality of high frequency (RF) function circuit elements are contained in a box thereof, which is partitioned into a plurality of areas by internal metallic walls for reducing possible electromagnetic interference among the function circuit elements.
Further, as another example, there is a transmitter-receiver reported in the Proceedings of the APMC98 Conference TU1A-3 “An Integrated Millimeter Wave MMIC Transceiver for 38 GHz Band” (in 1998). In this transmitter-receiver, a box thereof contains a plurality of transmitting-receiving MMICs and a dielectric substrate having passive circuits for connecting the MMICs, and the cover of the box is arranged to provide grooves for signal propagation.
Still further, as another example, there is a communication device reported in the Proceedings of the APMC98 Conference TU1A-1 “A Cost-Effective RF-Module for Millimeter-Wave Systems” (in 1998). This communication device is so structured that a box thereof contains a communication MMIC, a circuit substrate having passive circuits for connecting the MMIC, and a planar antenna.
Where a plurality of function elements are contained in a single box structure, physical distances among the function elements are shortened as the number of function elements is increased under condition that the size of the box is constant. Alternatively, with an increase in the number of function elements, it is required to increase the size of the box with respect to half of a free-space wavelength of a signal frequency (e.g., approximately 1.95 mm at 77 GHz). In either case, electromagnetic energy of a signal frequency emitted from a point of any function element or IC into the inside of the box readily propagates to another function element contained in the box through space therein, causing a variety of malfunctions due to electromagnetic coupling. For instance, in a transmitter-receiver or a millimeter-wave automobile radar transmitter-receiver module, if a part of a signal emitted from a transmitting function element into the inside of a box thereof propagates to a receiving function element contained in the box, there occurs such a trouble as saturation of a receiver circuit or increased noise in the receiver circuit. In particular, where the size of a box is considerably large with respect to half of a free-space wavelength of a signal frequency, a multiplicity of resonance frequencies signal components may be present in the inside of the box to worsen a condition of such a trouble as mentioned above.
Similarly to these phenomena, undesired energy emitted into the inside of a box readily causes undesired emission out of the box which unintentionally serves as a kind of antenna. On the contrary, external electromagnetic energy is likely to be received through the box, giving rise to a serious problem in requirements concerning EMI (Electro-Magnetic Interference) and EMC (Electro-Magnetic Compatibility).
To alleviate these problems, particularly electromagnetic interference in the inside of a box, a conventional communication device using such prior art as aforementioned has a box structure which is divided into a plurality of small areas by metallic partitions or a metallic box structure which provides local cut-off waveguides along signal paths for protection against undesired emission. In implementation of these conventional techniques, it is required to form a complex metallic box structure or to divide a high frequency passive circuit substrate into a plurality of sections. Further, due to formation of the complex metallic box structure or division of the passive circuit substrate, there occurs difficulty in mounting arrangement of semiconductor ICs and passive circuit parts, resulting in hindrance to mass-production of communication devices and reduction in manufacturing cost thereof.